Optical time shifter and routing system

ABSTRACT

An optical time shifter and routing system having high efficiency switched gratings to form optical time delay and routing networks. This system also includes a unique passive noise suppression device that can be used in multiple channel optical systems such as time shifters and routers to increase channel isolation and reduce crosstalk. The latter noise suppression devices are applicable broadly to the free-space time shifters as well as to other time shifters.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Contract Nos.F30602-94-C-0151 and F30602-95-C-0238 awarded by the U.S. Air Force. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to time delay systems, and, moreparticularly, to optical time shifter and routing systems whichincorporate the use of diffractive optics and noise suppressors.

BACKGROUND OF THE INVENTION

With the advent of substanial new uses for electro/optic systems, thereexists a greater need to effectively control the passage ofelectro-optic signals, both in their direction and time. This isespecially true in phased array systems, although, it should be realizedthat this is just one of numerous electro-optic systems which requirethe use of an optical time shifting and routing mechanism.

Phased array systems are generally made up of arrays of many relativelyisotropic radiators or emitters, spaced for example by half theirradiating wavelength, that are each driven coherently but with arelative phase (or time) shift among individual elements or amongsubarrays of elements. Controlling this phase shift across the array ofradiators permits the array to form a beam that is strongly peaked inthe far-field. Using this well established technique, the direction ofthe beam can be steered electronically (much faster than is possiblemechanically) by controlling the phase shifts. Further, the angularwidth of the beam decreases as the lateral extent of the arrayincreases--driving the need for extensive arrays (e.g., in excess of onehundred wavelengths lateral extent) and thus for large numbers ofelements. Even with subarraying techniques in which subsets of radiatorsare ganged to a common shifter so as to reduce the number of phaseshifters required, the requirement for rapidly introducing phase or timedelays into many parallel microwave channels forms a major technologicalchallenge. It would therefore be highly desirable to provide digitaltime or phase shifting of signals for each emitter in a fast, accurate,compact, lightweight, inexpensive system while introducing minimalinsertion losses and negligible spurious noise signals from scatter,reflections, and imperfect switch purity.

There are many practical barriers to implementing time delay networksdirectly in the microwave bands. These include difficulties such as: thepower splitters used in such networks are large; the cables orwaveguides used at microwave frequencies are bulky; the networks tend tobe lossy; and dispersion of these delay lines makes the use of multiplebands difficult. Photonic technologies can be applied to this problem,for example, by converting the microwave signals to modulation onoptical carriers, introducing the required delays optically, and thenconverting back into the microwave regime. This type of translationscheme permits the use of optical devices and techniques in phaseshifting that are superior to those operating directly in the microwaveregime.

For example, optical time delay networks can potentially be lightweight, compact, and insensitive to electromagnetic crosstalk andinterference. They can provide very long delays when required. Further,the dispersion effects are greatly reduced and multiple microwave bandscan use the same delay network. Still, the advantages of using opticalphase or time shifting must outweigh the overhead associated withconverting to and from the optical regime.

Another technological challenge associated with driving phased arrayantennas arises in systems utilizing large bandwidth signals. When beamforming is accomplished by introducing phase delays (rather than timedelays), large bandwidths cause the direction of the beam to detune fromits desired direction. Since very large bandwidths are required forcommunication and identification of targets and tracking (as opposed tomany searching tasks), true time delay beam forming networks are alsoimportant in high performance phased array systems.

In addition to the requirement for true time delay in beam forming forfuture phased array radar systems that utilize large bandwidth signals,it has been shown that time delay networks are needed with 1) lowinsertion loss (to reduce amplifier gain and resulting nonlinearitieswith strong signals); 2) low crosstalk among delay channels (to reduceamplitude and phase distortions in the resulting signal); and 3) lowspurious signal generation (to reduce the formation of unwanted lobes inthe array pattern). These additional requirements augment thetechnological challenge in future beam forming networks.

Recently there has been much attention to the application of photonictime delay networks for addressing the phased array beam formingproblem. There are many openings for the application of opticalapproaches in radar systems, and the concept of optical beam formingwith time delay networks has been physically demonstrated in W. Ng, A.A. Walston, G. L. Tangonan, J. J. Lee, I. L. Newberg, and N. Bernstein,"The First Demonstration of an Optically Steered Microwave Phased ArrayAntenna Using True-Time-Delay," Journal of Lightwave Technology, 9, 1124(1991). The bulk of the optical approaches, however, have been directedat switching and delaying the optical carrier using guided waves such asin optical fibers or planar waveguides. Many of these techniques usecombinations of integrated optical switches and guided wave delay lines.

Other approaches for optically introducing time or phase shifts whichhave difficiencies associated therewith include the use of heterodyningand coherent techniques and the use of segmented mirror spatial lightmodulators and polarization routing through prisms in free space.

It is quite apparent there is still much room for advancement in theseprior approaches, particularly with respect to losses, complexity,crosstalk, switch isolation, compactness and multiple reflectionsuppression.

It is therefore an object of this invention to provide an optical timeshifter and routing system which incorporates therein a free-spaceswitching technique.

It is a further object of this invention provide an optical time shifterand routing system which has superior switch isolation, multiplereflection and crosstalk suppression; less complexity and lowerinsertion loss; and less stringent wavelength tolerances than timeshifting systems of the past.

It is another object of this invention provide an optical time shifterand routing system which is extremely compact.

It is still another object of this invention provide an optical timeshifter and routing system which utilizes a series of switchablegratings therein.

It is still a further object of this invention provide an optical timeshifter and routing system which incorporates saturable absorber noisesuppressors therein.

SUMMARY OF THE INVENTION

The present invention overcomes problems associated with switchisolation, noise and crosstalk suppression, insertion loss, spuriousreflections, wavelength tolerance, and compactness that are present inother optical time shifter devices. The present invention includesdevices that use high efficiency switched gratings to form optical timedelay and routing networks. Also subject of this invention is a uniquepassive noise suppression device that can be used in multiple channeloptical systems such as time shifters and routers to increase channelisolation and reduce crosstalk. The latter noise suppression devices areapplicable broadly to the free-space time shifters as well as to othertime shifters and routers, for example, those using guided waves.

For a better understanding of the present invention, together with otherand further objects, reference is made to the following descriptiontaken in conjunction with the accompanying drawings, and its scope willbe pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the optical time shifter androuting system of this invention incorporating electrically switchablegratings therein;

FIG. 2 is a schematic representation of a typical saturable absorberutilized to suppress noise in the optical time shifter and routingsystem of this invention;

FIG. 3 is a schematic representation of a further embodiment of theoptical time shifter and routing system of this invention incorporatingpolarization switchable gratings therein;

FIG. 4 is a schematic representation of another embodiment of theoptical time shifter and routing system of this invention in which theoutput exits from a single location;

FIG. 5 is a schematic representation of an embodiment of the opticaltime shifter and routing system of this invention in which the outputexits from a single location through the use of a double passconfiguration;

FIG. 6 is a schematic representation of a further embodiment of theoptical time shifter and routing system of this invention in which theoverall size of the system is substantially reduced;

FIG. 7A is a schematic representation of the grating system utilized inthe embodiment of this invention shown in FIG. 6;

FIG. 7B is a chart representative of the various states of the gatingsystem of FIG. 7A.

FIG. 8 is a schematic representation of a further embodiment of theoptical time shifter and routing system of this invention in which amodulated signal is added to the optical carrier in order to allow thesimultaneous transmissioin of high and low frequency signals through thenoise suppression stage;

FIG. 9 is a schematic representation of an optical routing system ofthis invention utilized in conjunction with an auxiliary time delaynetwork;

FIG. 10 is a schematic representation of a single beam steering systemfor a phased array antenna in which another embodiment of the opticalshifter and routing system of this invention as shown in FIG. 11 can beincorporated therein;

FIG. 11 is a schematic representation of a further embodiment of theoptical time shifter and routing system of this invention for use withina phased array antenna;

FIG. 12A is a schematic representation of an asymmetric grating of thetype utilized in certain embodiments of this invention;

FIG. 12B is a schematic representation of a symmetric grating of thetype utilized in the embodiments of this invention shown in FIGS. 13-15;

FIG. 13 is a schematic representation of a further embodiment of theoptical time shifter and routing system of this invention whichincorporates symmetric gratings therein;

FIG. 14 is a schematic representation of a further embodiment of theoptical time shifter and routing system of this invention shown in FIG.13 which incorporates symmetric gratings therein and is configured in acompact design utilizing a single set of gratings;

FIG. 15 is a schematic representation of a further embodiment of theoptical time shifter and routing system of this invention which is evenmore compact than the configurations shown in FIGS. 13 and 14; and

FIG. 16 is a schematic representation of a further embodiment of theoptical time shifter and routing system of this invention in which aplurality of such devices are stacked using micro-optical techniquesinto a compact form enabling the independent time shifting of aplurality of input carrier beams.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a digital free space optical time delay(shifter) and routing network utilizing a switchable grating basedapproach together with novel noise suppression techniques. Theincorporation of free space switching in the present invention hasseveral distinct advantages over past time delay techniques. Morespecifically, these advantages include the potential for superior switchisolation, multiple reflection and crosstalk suppression, lesscomplexity and lower insertion loss, and less stringent wavelengthtolerances. Furthermore, the utilization of free space geometry enablesthe present invention to utilize saturable absorber noise suppresserswithin the system. This technique provides a very large gain and signalpurity by suppressing crosstalk, scatter, and multiply-reflected(spurious) waves.

Reference is now made to FIG. 1 of the drawings which illustrates thebroad concept of the invention in schematic fashion, thereby presentingan overview of the digital optical time shifter and routing system ofthe present invention in one of numerous embodiments, the otherembodiments being set forth below with respect to the remaining figures.

One embodiment of the optical time shifter and routing system 10 of thisinvention is illustrated in FIG. 1 of the drawings and depicts aplurality (four being illustrated therein as an example, not as alimitation on this invention) of switchable gratings 12, 14, 16, 18which are controlled by any suitable control signals C1-C4 and shown ina cascading fashion. This embodiment enables a beam of electromagneticradiation, preferably in the form of an optical input carrier 20 tofollow 16 possible optical paths. All of these paths being configured infree-space, without external limitations, although it should be realizedthe system of this invention can be encompassed in a medium ofdielectric constant other than unity. In other words, glass or othertransparent slabs may be used to seperate the planes of switchedgratings to provide for a monolithic and stable device that is noteasily misaligned. Thus, free space in the context of this inventiondescribes the use of freely propagating electromagnetic waves incomparison to guided wave systems in which the optical carriers areconfined to waveguides which are typically of fiber or planar form.

In this embodiment of the present invention, each optical path can add apreselected constant increment in time delay to a transmitted signal. Byselecting one of the 16 combinations of "on or off" states in thegratings 12, 14, 16, and 18, an input optical carrier signal 20emanating from any suitable source of electromagnetic radiation (notshown) is diffracted to follow one of the series of paths as shown inFIG. 1. For example, the beam of electromagnetic radiation or opticalcarrier 20 follows either path A or B after passing through grating 12and thereafter after passing through grating 14 follows preselectedpaths C, D, E, or F. Thereafter further different optical paths may befollowed as illustrated in FIG. 1.

More specifically, the time delays are strong functions of parametersincluding stage separation 21 and diffraction angle 23. The gratings,which may take the form of holographic elements, have a distincttradeoff relating to the Bragg regime of their operation. Parameterssuch as thickness and spatial period must be balanced to assure highdiffraction efficiency but also to retain maximal angular and spectralbandwidth tolerances. Finally, the use of micro-optics enables verycompact time shifters, and requires the balancing of channel density anddiffractive crosstalk effects as will be pointed out hereinbelow.

Two major free parameters in the time shifter configurations are theminimum spacing of the stages and the bias diffraction angle 23. As thenumber of stages are increased, their separation 21 increases inmultiples so it is usually advantageous to keep spacing small, thuskeeping the overall shifter compact. Values of stage separation 21 inthe range from 1 mm to 10 mm are small and at the same time easy toengineer. A six-bit time shifter as shown in FIG. 1 would only need beon the order of 3 cm in length. The range in digitally selectableminimum time increments available in such a shifter is a strong functionof bias diffraction angle. The more stages or larger the diffractionangle, the longer is the maximum delay that is obtainable.

An advantage of working in the free-space domain in the presentinvention is the ability to trade off between thin (Raman-Nath)diffraction regimes and thick (Bragg) diffraction regimes. In the thinlimit, diffraction efficiencies are low but there are practically nowavelength or angle restrictions on the diffraction efficiency. At theother extreme, Bragg diffraction is characterized by extremely highdiffraction efficiency but is restricted to narrow angular and spectralbands. The trading-off between the "thin" (Raman-Nath) and "thick"(Bragg) diffraction regimes removes a difficulty with many laser sourcessince moderate frequency drifts and mode hops (e.g., tens of nm) willnot cause a large change in diffraction or coupling efficiency. Thisreduces a dispersion problem faced in many guided wave approaches.

The time shifting and routing systems of this invention generallyutilize volume phase diffraction gratings that permit switching of theincident energy between two or more of the diffracted orders. Theprimary mechanisms considered which obtain this diffracted-orderswitching are electrical switching, optical switching, and polarizationswitching of the gratings, all of which are discussed with specificembodiments of the invention.

As shown in FIG. 1, the diffractive optical gratings are switchedelectrically to redirect the beams. An advantage of electrical switchingis that the gratings can be small and are potentially scalable toaccommodate large arrays. The optical, electrical, polarization andother switching mechanisms are all potentially useful for the opticaltime shifter and routing system of the present invention, while withcurrent technology, the polarization-based mechanisms appear to beparticularly useful.

Recently it has been demonstrated in the literature that high efficiencyvolume diffraction gratings which are recorded in permeable media, suchas the DMP-128 photopolymer manufactured by Polaroid Corporation,Cambridge, Mass. can be made to be rapidly switchable between high andlow diffraction efficiency states under electric control by imbibing thestructure with nematic liquid crystals. In this technique, the crystalsare rotated by the applied electric field and their refractive indexswitches between ordinary and extraordinary values. By choosing thematerials so that one of these switchable values of refractive indexmatches that of the phase modulation in the grating, the gratingmodulation is effectively switched "off-and-on" as the liquid crystal"fill" material index matches and mis-matches the modulation,respectively.

Most of the optical time shifter and routing systems described hereincan utilize electrically switched gratings. In this approach the volumephase diffraction grating, which is typically on the order of 10 micronsthick, is bounded by transparent conducting electrodes.

Referring once again to FIG. 1, it should be further understood thatalthough four such gratings are illustrated in the figure, the number ofgrating can vary in accordance with the utilization of this invention.For example, with the diffraction efficiency of all four gratings off,the shortest, straight through path (that is, following along opticalpath A, C, etc.) is selected. If the first grating 12 is "on", and allothers "off," the longest path is selected following optical path B, F,etc. Consequently, by a combination of "on", "off" signals applied tothe gratings, it is possible in free space to not only route the opticalsignal to a desired output area, but also to do so with a predeterminedtime delay.

Continuing with a description of the embodiment of the invention shownin FIG. 1, after passing through the last grating 18, the chosen opticalpath can be directed by means of any suitable fixed optical redirectioncomponent 22, such as a diffractive microlens array which is utilized tomake the direction of the output beams from the many channels uniform.For example, as shown in FIG. 1, the off-axis holographic lenses switchoff-axis beams on-axis prior to impinging upon a suitable detector 24.In this latter example a focusing effect (optical power) is introducedinto the off-axis elements to interface to the saturable absorbercrosstalk suppressor. Alternatively, an additional switched grating maybe used in place of the fixed optical redirection component 22 since thevarious channels are either directed in the proper ion axisi direction,or in a discrete off-axis direction. Accordingly, the switched gratingis turned off if the selected channel is one exiting the precedinggrating cascade in the desired direction, and is turned on if theselected channel is off axis, thus diffracting the beam in the desireddirection.

Another important aspect of the present invention is the incorporationtherein of a saturable absorber nois suppressor 26 interposed betweenthe optical redirection component or difractive microlens array 22 anddetector 24. This arrangement with the use of high efficiency volumediffraction gratings permits low insertion loss, and by focusing eachchannel or optical path through a saturable absorber or other noisesuppresser stage 26 spurious, scattered, and ghost signals can begreatly attenuated with little loss of power in the selected channel.The details of this suppressor stage or saturable absorber 26 will beexplained in greater detailed hereinbelow.

The saturable absorber noise suppressor 26 utilized in optical timeshifter and routing systems of this invention may use any saturableabsorber material, for example, nonlinear absorption in atomic,molecular or tailored quantum-well saturable absorbers to heavilyattenuate signals in non-selected channels while imposing lower ornegligible loss on the signal in the selected channel. Each of theselectable optical paths is focused on a separate location of asaturable absorber film. The nonlinear absorber is optimized so that thepath that is selected has enough intensity to saturate the absorber, andis transmitted with little effect. However scatter, imperfectdiffraction efficiency and switching contrast, and other effects combineinevitably resulting in noise signals that propagate in the nonselectedchannels which have the improper delay or position. These signals can bethe cause of many deleterious effects in the overall system. While it isimpossible to perfectly eliminate all noise signals, the presentinvention is capable of keeping them at a relatively low signal level incomparison with the selected channel. When these low intensity beams areincident on the absorber 26, they are not capable of saturating theabsorption, and are much more strongly attenuated than the beam in theselected channel.

Reference is made to FIG. 2 for a schematic illustration of theoperation of absorber 26. The effectiveness of the absorber involves theselection of a nonlinear material with the proper (optimized) saturationintensity and time constant. The required intensity varies with the timeconstant of the nonlinear material, and the time constant must be chosento be fast enough to not limit switching time, and yet slow enough so asnot to distort the modulation on the optical carrier.

Another novel feature of the optical time shifter configurations of thisinvention is their use of a device with thin saturable absorber films toabsorb any weak signals that leak through non-selected channels due toimperfect diffraction efficiency, scatter, etc.--while transmitting thesignal in the selected path with only very small attenuation. The basicconcept is shown in FIG. 2, and according to this technique, the opticalsignals in each of the selectable delay paths are focused onto a thinfilm of saturable absorber before the optical channels are recombined ordetected (reconverted to microwave channels). The saturable absorbernoise suppressor (SANS) is a scatter, spurious beam, and crosstalksuppression device that was developed for use in the time shiftersdescribed here, but which are useful for enhancing signal-to-noise in abroad range of multichannel optical systems.

More specifically, a saturable absorber responding to incident intensityI has a transmittance given by:

    T=e.sup.-α(l)(L),

where L is the thickness of the saturable absorber and the intensitydependent absorption is given by: ##EQU1##

The saturation intensity is I_(sat) =hν/στ, where α_(o) is the lowintensity absorption coefficient, and σ is the absorption cross sectionof the absorber.

For a typical saturable dye which is assumed, for example, to have aresponse time t of a microsecond, the saturation intensity is of theorder 10³ W/cm². If the beam is focused to a spot size of 5 microns, thepower needed to saturate the absorber is on the order of a milliwatt.Thus, such a film of saturable absorber with a frequency response on theorder of a megahertz can be used to significantly attenuate the lowintensity signals that arise from scatter or that leak through nonselected paths--and yet transmit the desired signal, even at powerlevels as low as milliwatts, with little attenuation. Alternativelyhigher powers can be used in the switched grating optical time shifterof this invention and the need for focusing can be eliminated (thusreducing complexity). By choosing the saturable absorber time constantto be slower than the modulation frequency on the optical carrier,signal degradation effects resulting from the saturable absorber will beavoided.

A key tradeoff should be possible wherein a proper absorber timeconstant can be matched with a useful saturation intensity. The timeconstant must be slow enough to pass desired modulations on the opticalcarrier without distortion, and fast enough so as not to limit beamsteering (channel selection) time. For example, if the absorber timeconstant is on the order of the inverse of the RF modulation frequency,the absorber could recover in a period when the modulated carrier wasrelatively low in intensity, and would then absorb the leading edge ofthe following higher intensity signal while it became re-saturated. Atthe other end of the tradeoff, if the absorber time constant is slowcompared with the electronic or optical switching time, an additionaltime delay will be required once the new channel is selected while theabsorber saturates--thus limiting the response time of the time shifter.However, keeping the saturation intensity as low as possible isdesirable since the lower saturation intensities result in morepractical required power levels, such as 1 mW per carrier and below.

Saturable absorbers have the advantages of being relatively inexpensive,available in a broad range of saturation intensity/time constants, andare often easily incorporated into convenient solid state hosts such asacetate sheets. Any absorbing material or system that saturates withintensity can be applied in this device. One type of s particularinterest with the present invention is multiple quantum well (MQW)saturable absorbers. Dyes, ions, and complexes also be applied. Thesaturation intensity typically decreases as the time constant of thesaturable absorber increases, in what is a well defined trade-off formany saturable absorber systems. It appears possible to deviate fromthis usual trade-off through suitably designed multiple quantum wellabsorbers that can provide faster time responses at a given saturationintensity than can most other saturable absorber systems. Theimprovement in required intensity for a given time constant shown bytypical MQW saturable absorbers is on the order of three orders ofmagnitude.

Saturable absorber noise suppressors of the present invention do notrely on the absorbers being localized in thin films. For example, asaturable absorber noise suppressor can be fabricated in a waveguidesystem by doping a small concentration of the absorber in either theguide or clad region, or both.

In summary, the saturable absorber noise suppressor 26 makes use of anonlinear, intensity dependent absorption to attenuate spurious signalsin non-selected delay channels while imposing smaller or negligibleabsorption on the optical carrier in the selected delay channel.

Once again, referring to FIG. 1 of the drawings, the delayed carriersignal 20 after passing through absorber 26 is then detected at detector24, or in a useful extension of this present configuration, a symmetricmirror image of this configuration in order to exit the delayed paths ata single location as illustrated in FIG. 4 of the drawings, for example.The wide latitude in choice of grating spacing diffraction angle, andother parameters gives fine control of digital time increments from thefemtosecond regime through hundreds of picoseconds and nanoseconds withlatger configurations. Total delays of 2^(n) times these increments areobtainable with the configuration shown in FIG. 1 of the drawings.

Utilizing n cascaded gratings as shown in FIG. 1, gives digitalselection among 2^(n) delays which range from 0 to 2n-1 times theincremental delay time. Therefore the more gratings or stages, or thelarger the diffraction angle 23, the longer the maximum delayobtainable.

Reference is now made to FIG. 3 of the drawings, which utilizes apolarization switched grating technique. The polarization switchingapproach as illustrated in FIG. 3 of the drawings is incorporated withinan optical time shifter and routing system 30 similar to theconfiguration shown in FIG. 1 but incorporates cascades of polarizationrotating switches 32, 34, 36 and 38. These devices can be made up of aferroelectric liquid crystal polarization switch 40 in combination witha polarization sensitive holographic grating 42. With the embodiment ofthe invention shown in FIG. 3 of the drawings, the switching is stillelectrical, but a control signal is used to switch the diffracted ordercontaining the optical carrier by either leaving the current state ofpolarization intact or rotating it by 90 degrees.

A subtle distinction is made in mapping the control signals for a givendelay channel selection depending on whether direct electrical orpolarization switching is used. In the former case, each grating is adiffractive or transmissive state depending on the control signal. Thusthe control sequence 1-0-0-0 corresponds to the lower most path which isdiffracted at the first grating and transmitted through all thesubsequent gratings in the zero-order. However, assuming the incidentcarrier is in the non-diffractive polarization state and thepolarization switching mechanism, the same sequence could represent thefirst rotator being active and the others being off, in which case theoptical carrier would be diffracted by the first and all subsequentgratings, thus taking the sixth delay channel from the bottom of thefigure.

As with a configuration shown in FIG. 1 of the drawings a similarsaturable absorber noise suppressor44 is utilized to suppress noise,crosstalk, reflections, and spurious signals resulting from imperfectswitching purity. Also, the polarization switched grating technique canbe utilized with other embodiments of the present invention.

Reference is now made to the embodiment of FIG. 4 of the drawings inwhich the free space digital optical switched grating time shifter androuting system of the present invention takes on a symmetricconfiguration by reflecting it and unfolding it about the saturableabsorber noise surpresser 52. The time shifter and routing system 50 asshown in FIG. 4 has the practical advantage that the optical carrier 20enters and exits at a single spatial location, regardless of theselected amount of delay. With the embodiment illustrated in FIG. 4 ofthe drawings, a set of recombination gratings 54, 56, 58 and 60 areutilized in combination with the gratings 62, 64, 68, and 70 aspreviously discussed with respect to FIG. 1 of the drawings. Thisrecombination set of gratings may be controlled by the same signals asthe initial set of gratings and so only n control signals are requiredfor the n bit selection in delays. Therefore only a single detector isrequired.

At first glance, it appears that the saturable absorber noise suppressor52 located in the center of the configuration may be effective only forsuppressing noise and spurious beams generated in the front half of thesystem. However, this location is effective for eliminating thedeleterious beams from the second half of the system too. For example ifchannel 1-0-0-0 is selected, spurious signals propagating into the other15 channels, if not blocked in the center, could follow many paths inthe combination stage of the system to arrive superimposed on theoptical carrier with the selected delay. All fifteen of these spuriouspaths are, however, attenuated by the saturable absorber noisesuppressor 52 in the center of the system. On the recombination side,however, the one selected channel is free to couple to other diffractedorders through imperfect switch isolation, for example. But since thereis a unique path from any of the channels in the center to the one exitlocation, any such spurious beams will not be superimposed on theoptical carrier with selected delay.

A double pass configured time shifter and routing system 80 isillustrated in FIG. 5 of the drawings. This double pass configuration isextremely compact and has the option to double pass the optical orelectromagnetic beam through a grating cascade rather then unfolding itabout the saturable absorber noise suppressor as shown in FIG. 4.

This embodiment of the invention is very useful in certaincircumstances. For example, the system can cost nearly half as muchsince half the hardware is required. The added compactness is also adistinct advantage. The time shifter and routing system 80 incorporatestherein a plurality of gratings 82, 84, 86 and 88 in combination with adifractive microlens array 90. The lens array 90 images the output on areflective element 92, in the form of, for example, a mirrored surface.In this instance the saturable absorber 94 may be coated directly on themirror 92, with the effect being of using a "black mirror" in the middleof the system which is nearly transparent only in the tiny "pinhole"that it is bleached by the high intensity of the carrier electromagneticbeam in the selected channel or path of operation. The double passthrough the saturable absorber 94 in this embodiment of the inventiongives further absorption to weak (spurious) beams while enhancing thebleaching of the absorber and location of the selected optical channel.

In order to differentiate between the incoming electromagnetic beam orinput carrier 20 and the outgoing electromagnetic beam 96, any suitablebeam splitter 98 is utilized in conjunction with a quarter-wave plate 99at the input end of the system. This arrangement permits the output totravel in one direction while the input travels in another. As with allof the other configurations, the specific type of grating describedhereinabove utilized with the present invention can vary within thescope of the present invention. For example, the polarization switchedgratings may be utilized in all of the embodiments, if so desired.

Reference is now made to FIG. 6 of the drawings which schematicallyillustrates a further embodiment of the present invention in which thetime shifter and routing system 100 incorporates therein a series ofclosely cascaded gratings which act in the tri-state. In other words, asshown in FIG. 7 of the drawings, the cascaded gratings or multi-stategrating 102 incorporate therein a pair of gratings 104 and 106 which areindependently switchable and can be fabricated with independentdiffraction angles. Since the gratings typically operate in theQuasi-Bragg regime, if the first one is turned "on", the beam can beBragg-mismatched through the second element, so the second control inthis case is a "don't care". The ability to select from among multipleangles can greatly compact the shifter and routing system of the presentinvention.

Referring once again to FIG. 6 of the drawings, multi-state gratings 110and 112, for example, are arranged in a similar fashion to the gratingsas shown in FIG. 4 of the drawings. In such an arrangement, an inputelectromagnetic beam of electromagnetic radiation which is received bygrating 110 can follow any one of three different paths, A, B, or C.And, thereafter can either be further divided into paths D, E, F, or G,H, I, or J, K or L as shown in FIG. 6. As with the embodiment of theshifter and routing system shown in FIG. 4 of the drawings, a saturableabsorber noise suppressor 116 is utilized in conjunction with a pair ofdifractive microlens arrays 118 or other suitable redirection componentsuch as discussed with FIG. 1. In this embodiment the optical carrier isoutput at a single location, similar to that shown in FIG. 4.

A motivation for use of this multi-state grating configuration withinanother embodiment of this invention is that simple higher diffractedorders of a single frequency grating cannot, in general, be used forthis application and provide equally spaced delay increments. This isbecause higher grating orders add equal increments in the sine of thediffracted angle. Thus incremental deviation angles increase from orderto order, which is the opposite of the decreasing increments needed toproduce equal time increments. For example, if the first order isdiffracted at 30 degrees, an incremental change of 10.2 degrees (to 40.2degrees) would be useful for the configuration shown in FIG. 6 for equaldelays. Similarly, the next properly spaced order would be increased by6.7 degrees.

Reference is now made to FIG. 8 of the drawings which clearly schematicillustrates a simultaneous microwave and audio transmission time shifterand routing system 120.

There are two special cases where signal distortion may be encounteredwith the saturable absorber noise suppressor utilized within the presentinvention. The first occurs when very large amplitude modulation isimposed on the optical carrier, and a series of "low" bits or long lowcarrier intensity occurs. In such a case if the saturable absorber timeconstant is exceeded, the absorption will begin to rise and thefollowing signal edge will be distorted as the leading intensity edgedrives population into the higher level and the absorber isre-saturated. The second case where this problem might occur is if a lowfrequency (e.g., audio) modulation is imposed on top of the opticalcarrier. Again, if the modulation is large, recovery of the absorber canoccur with subsequent signal distortion. An interesting solution to thisproblem is also recognized in the present invention with theintroduction of an ancillary pulsed or CW beam to the incoming signal,as illustrated in FIG. 8 of the drawings. This added beam can keep thesaturable absorber noise suppressor bleached for the selected channel orpath, while not affecting the non-selected channels appreciably. For theCW case, the resulting bias can be subtracted; and in the modulated casethe single modulation frequency can be filtered either optically orelectronically from the desired time-shifted signal.

Another embodiment of this invention is illustrated in FIG. 9 of thedrawings in which the present invention is utilized as a digital opticalspatial router. In lower frequency or large aperture true time delayapplications, delays of many nanoseconds may be required. Since the timedelay created in these devices is due to additional optical paths thatare digitally added to the optical carrier, delays in the nanosecondregime can cause the configuration that is shown in FIG. 1 to becomephysically large in two dimensions (although using multiple angles asshown in FIG. 6 can help maintain compactness). The constant delayoptical carrier router 140 shown in FIG. 9 of the drawings is useful inthis long delay regime. In this example the n cascaded switchablegratings 142 are used to select among 2n spatial locations whereconventional delay lines or an external delay network 144 may beintroduced. For example, if an array of 16 variable length fiber delaylines are located in delay network 144 after the free-space opticalrouter 140 the optical carrier 146 may be routed to the desired fiberwith proper setting of the 4 digital control signals 148. By introducingthe saturable absorber noise suppressor 150 as described above in thiscoupling plane or in an ancillary image plane, the benefits of enhancedswitch purity and crosstalk suppression of the free-space concept ofthis invention is retained.

The concept of an entire array driver utilizing the switching gratingtime shifter and routing system of the present invention is illustratedin FIGS. 10 and 11 of the drawings. More specifically, the free-spacedomain has several degrees of freedom that are more highly constrainedor nonexistent in most guided wave approaches. These include the abilityto readily multi-pass the configurations and to interface to othertechnologies. These cases are illustrated with respect to FIGS. 10 and11.

For example, when a phased array 160 as shown in FIG. 10 is used to forma single beam at a given direction, the phase delays between each of theadjacent radiators 162 (when regularly spaced along a plane) are allidentical. For this case, an unfolded version of the time shifter androuting system as shown in FIG. 1 can be used to drive an entire array,or at least to drive a number of contiguous elements in a subarray. Thiscase, illustrated by the embodiment of the time shifter and routingsystem 170 shown in FIG. 11 of the drawings, depicts an optical carrier172 propagating through the selected path as determined by theswitchable gratings 174 and exits at a single spatial location 176. Thissignal is detected by a detector 178 to provide the signal with timedelay t, and a portion 180 of the delayed signal is then amplified byamplifier 182 and re-transmitted through the same optical time shifter170, giving a signal with delay of 2τ plus a constant bias delay thatcan be compensated for. The process is continued to provide a series ofsignals with integral multiples of the digitally selected delayincrement. These signals can be used to drive neighboring radiators 162across a subset of an entire phased array using only a singleconfiguration.

Consider a planar, 1-D phased array 160 as illustrated in FIG. 10. Thebasic principle on which this configuration is based is that when thebeam from an array is steered to a specific direction, the signal toeach of the array elements 162 is shifted in phase (or time) withrespect to its neighbor by an identical delay. In FIG. 10, the biasdelay to the first element is an arbitrary quantity T, and is not afactor in the array operation.

However when the array is steered at a given angle, the delay betweeneach of the neighboring radiators is t, and thus the original signalmust be obtained with integral multiples of this steering delay τ, i.e.,T, T+τ, T+2τ, T+3τ, etc. In such a case, independent control of thephase shift to each element, as provided by a gang of many phaseshifters (one for each array element) is overkill and not required.Rather, the series of delayed signals may be obtained by multiplepassing the original signal through the single switchable delay line, asshown in FIG. 11.

The detection, amplification, and rebroadcast can be accomplished in acompact space with "smart pixel" technology where, for example, adetector, amplifier, and microlaser are located in a cell that isreplicated in a dense array. Naturally noise can be a major limitationdue to the multiple passes, since with repeated detection andregeneration as shown in FIG. 11 the noise accumulates--and thereforedriving a vary large array in this fashion may be impractical. Howeverapplications such as backpack satellite communications require only arelatively small number of elements, and this configuration may beuseful for the entire array. These configurations can be used to driveentire arrays with small numbers of elements, or several suchconfigurations can be used to drive subarray regions of a larger arrayto reduce the noise limitations. One such entire array driver is capableof steering a small array in a single dimension, and two can be used for2-D steering.

The noise from cascaded detection-regeneration cycles can be avoidedthrough optical amplification of the carrier after each pass or afterevery few passes through the system. This option would likely extend thenumber of passes that could be obtained for a given noise level, sincecascaded detection and regeneration can be avoided entirely or reduced.Bias time delays for each pass can be equalized in optical fibers orwiring. For wide angle, 2-D steering, 2 or four units may be requireddepending on antenna configuration.

In another preferred embodiment of the subject invention, symmetricgratings are used to form time shifters that have improved compactnessand possess many practical advantages. An asymmetric volume grating 200is shown in FIG. 12A, where the Bragg planes 202 are tilted and theBragg resonant incident beam 204 and diffracted beam 206 form unequalangles with respect to the grating surface normal. For the case of asymmetric volume grating 210 shown in FIG. 12B, the Bragg planes 212 arenot tilted and the Bragg resonant incident beam 214 and diffracted beam216 are symmetric with respect to the grating surface normal. Since theBragg planes are not tilted, the fabrication of the gratings is simplerto optimize as the Bragg plane angle does not vary with shrinkage orexpansion of the volume media through processing. The deviation anglebetween the incident and diffracted beams in the symmetric grating casecan be larger, resulting in more compact configurations. Other systemadvantages are realized with the symmetric angles since the uniformangles of the various channels are the same throughout the system, whichtends to equalize surface losses, dispersion, and retardance variationif polarization switching is used, etc.

Reference is now made to FIG. 13 of the drawings which schematicallyillustrates a further embodiment of the present invention in whichsymmetric switchable gratings are used. Here symmetric recombinationgratings 226, 228, and 230 are used in combination with symmetricgratings 220, 222, and 224 as previously discussed with respect to FIGS.1 and 4 of the drawings. A saturable absorber noise suppressor stage 234is incorporated in the embodiment as before to decrease the spurioussignal levels in the non-selected channels.

Reference is now made to FIG. 14 of the drawings which schematicallyillustrates a further embodiment of the present invention in whichsymmetric switchable gratings are used in a more compact system. Heresymmetric switchable gratings 250, 252, and 254 are used both fordefinition of the various delay paths and for recombination of thepaths. Thus only one set of switchable gratings is required, while stillproviding for a single output beam location. The crosstalk suppressor264 is shown with integral redirection elements such as fixed gratings.

Reference is now made to FIG. 15 of the drawings which schematicallyillustrates a further embodiment of the present invention in whichsymmetric switchable gratings are used in a still more compact system.Here symmetric recombination gratings 276, 278, and 280 are used incombination with symmetric gratings 270, 272, and 274 as previouslydiscussed with respect to FIGS. 1 and 4 of the drawings. Fixedredirection components 284 are used to wrap the configuration in acompact space. In FIGS. 13-15, optical redirection components 232, 262,and 282 are incorporated as described with component 22 of FIG. 1.

The ability to miniaturize the optical time shifter configurations ofthe present invention is critical to success in many applications. Usingmicro-optical techniques as shown in FIG. 16, many independent delaychannels can be packaged in close proximity. A key limit on packingdensity is the optical channel (beam) diameter required to controloptical crosstalk due to diffraction spreading. The longer the channelshave to propagate in free space, the larger the width of the channelsmust be. As a rule of thumb, the channels must have a width D given by:##EQU2## where λ is the wavelength and Z is the required free spacepropagation distance.

For example, consider a 5-bit cascade with stage separations of 1, 2, 4,and 8 mm and a diffraction angle of 45 degrees. The maximum channellength is 21.2 mm, and at a wavelength of 1.3 microns, the diffractionlimited channel spacing would be approximately 0.17 mm. In that case 6channels could be vertically stacked in a 1 mm height with a transversedimension that is slightly larger than 1/2" square. This packing densitytradeoff is still very good as, from diffractive crosstalkconsiderations only, a 1/2" cube should allow the containment of 766-bit shifters.

The miniaturezed micro-optic packaging of many independent optical timeshifter configurations 300 is illustrated in FIG. 16. Here m opticaltime shifters or routers are stacked on top of another. Time shifter 302is stacked on top of shifter 304, and so on to the m'th shifterconfiguration 308. The vertical height of each stacked layer is given byD as defined above. The optical carrier inputs 312, 314, and 318 for the1'st, 2'nd, and m'th shifters, respectively, form a vertical column thatcan be interfaced to a linear array of microlasers with driveelectronics.

Although the invention has been described with reference to particularembodiments, it will be understood that this invention is also capableof further and other embodiments within the spirit and scope of theappended claims.

What is claimed is:
 1. An optical time shifter and routing systemcomprising:first switchable diffractive means for receiving a beam ofelectromagnetic radiation and for selectively directing said beam intoat least first and second optical paths; second switchable diffractivemeans interposed within said optical paths for receiving said beam ofelectromagnetic radiation and selectively directing said beam ofelectromagnetic radiation following said first optical path into atleast third and forth optical paths, and for selectively directing saidbeam of electromagnetic radiation following said second optical pathinto at least fifth and sixth optical paths; each of said optical pathsbeing of a different predetermined length; and means operably associatedwith said first and second means for controlling the path taken by saidbeam of electromagnetic radiation.
 2. An optical time shifter androuting system as defined in claim 1 further comprising:means fordetecting said beam of electromagnetic radiation after having passedthrough a predetermined combination of said optical paths; and meansoptically aligned adjacent said detecting means for directing said beamof electromagnetic radiation after having passed through a predeterminedcombination of said optical paths to said detecting means.
 3. An opticaltime shifter and routing system as defined in claim 2 further comprisingmeans interposed between said directing means and said detecting meansfor suppressing unwanted signals resulting from the passage of said beamof electromagnetic radiation through said predetermined combination ofsaid optical paths.
 4. An optical time shifter and routing system asdefined in claim 1 further comprising a plurality of additional beamreceiving and directing means for permitting said beam ofelectromagnetic radiation to follow a predetermined number of furtherdifferent optical paths whereby, based on the combination of opticalpaths through which said beam of electromagnetic radiation passes, thetime said beam takes to pass therethrough is controlled.
 5. An opticaltime shifter and routing system as defined in claim 1 wherein each ofsaid receiving and directing means comprises a diffractive elementcontrolled by an electrical signal.
 6. An optical time shifter androuting system as defined in claim 5 wherein said diffractive element isin the form of a diffractive grating.
 7. An optical time shifter androuting system as defined in claim 3 wherein said signal suppressingmeans is in the form of a saturable absorber using nonlinear absorptionto heavily attenuate said unwanted signals while imposing negligibleattenuation to the desired signal permitting it to pass onto thedetecting means.
 8. An optical time shifter and routing system asdefined in claim 3 wherein said signal suppressing means is in the formof a saturable absorber using nonlinear absorption in quantum-wellsaturable absorbers to heavily attenuate said unwanted signals whileimposing negligible attenuation to the desired signal permitting it topass onto the detecting means.
 9. An optical time shifter and routingsystem as defined in claim 2 wherein said directing means comprises adiffractive microlens array.
 10. An optical time shifter and routingsystem as defined in claim 4 wherein each of said receiving anddirecting means comprises a polarization switched grating, said gratingcomprised of the combination of a ferroelectric liquid crystalpolarization switch and a polarization sensitive holographic grating.11. An optical time shifter and routing system as defined in claim 1wherein said optical paths are created in free space devoid of externalboundaries.
 12. An optical time shifter and routing system as defined inclaim 3 further comprising a plurality of additional beam receiving anddirecting means for permitting said beam of electromagnetic radiation tofollow a predetermined number of further different optical pathswhereby, based on the combination of optical paths through which saidbeam of electromagnetic radiation passes, the time said beam takes toreach said detector is controlled.
 13. An optical time shifter androuting system as defined in claim 12 wherein each of said receiving anddirecting means comprises a diffractive element controlled by anelectrical signal.
 14. An optical time shifter and routing system asdefined in claim 13 wherein said optical paths are created in free spacedevoid of external boundaries.
 15. An optical time shifter and routingsystem as defined in claim 1 wherein switchable diffractive means are inthe form of symmetric switchable gratings.
 16. An optical time shifterand routing system comprising:a first plurality of switchablediffractive means for receiving a beam of electromagnetic radiation froma first location and directing said beam along a plurality of differentoptical paths; each of said optical paths being of differentpredetermined lengths; said beam of electromagnetic radiation capable ofbeing output from one of a plurality of different second locations; asecond plurality of switchable diffractive means for receiving said beamof electromagnetic radiation being output from one of said plurality ofdifferent second locations and directing said beam along a plurality ofdifferent optical paths to a third location; and means operablyassociated with said first and second plurality of means for controllingthe path taken by said beam of electromagnetic radiation; whereby, basedon the combination of optical paths through which said beam ofelectromagnetic radiation passes, the time said beam takes to go betweensaid first and third location is controlled.
 17. An optical time shifterand routing system as defined in claim 16 further comprising meansoptically aligned adjacent said second locations for directing said beamof electromagnetic radiation after having passed through a predeterminedcombination of said optical paths to said second plurality of receivingand directing means.
 18. An optical time shifter and routing system asdefined in claim 17 wherein said directing means is in the form of adiffractive microlens array.
 19. An optical time shifter and routingsystem as defined in claim 17 further comprising means interposedbetween said directing means and said second plurality of receiving anddirecting means for suppressing unwanted signals resulting from thepassage of said beam of electromagnetic radiation through saidpredetermined combination of said optical paths.
 20. An optical timeshifter and routing system as defined in claim 16 wherein each of saidreceiving and directing means comprises a diffractive element controlledby an electrical signal.
 21. An optical time shifter and routing systemas defined in claim 20 wherein said diffractive element is in the formof a diffractive grating.
 22. An optical time shifter and routing systemas defined in claim 19 wherein said signal suppressing means is in theform of a saturable absorber using nonlinear absorption to heavilyattenuate said unwanted signals while imposing negligible attenuation tothe desired signal permitting it to pass onto the detecting means. 23.An optical time shifter and routing system as defined in claim 19wherein said signal suppressing means is in the form of a saturableabsorber using nonlinear absorption in quantum-well saturable absorbersto heavily attenuate said unwanted signals while imposing negligibleattenuation to the desired signal permitting it to pass onto thedetecting means.
 24. An optical time shifter and routing system asdefined in claim 16 wherein each of said receiving and directing meanscomprises a polarization switched grating, said grating comprised of thecombination of a ferroelectric liquid crystal polarization switch and apolarization sensitive holographic grating.
 25. An optical time shifterand routing system as defined in claim 16 wherein said optical paths arecreated in free space devoid of external boundaries.
 26. An optical timeshifter and routing system as defined in claim 17 wherein said opticalpaths are created in free space devoid of external boundaries.
 27. Anoptical time shifter and routing system comprising:first means forreceiving a beam of electromagnetic radiation from a predeterminedlocation and for selectively directing said beam into at least first,second and third optical paths; second means interposed within saidoptical paths for receiving said beam of electromagnetic radiation andselectively directing said beam of electromagnetic radiation followingsaid first optical path into at least forth, fifth and sixth opticalpaths, for selectively directing said beam of electromagnetic radiationfollowing said second optical path into at least seventh, eighth andninth optical paths, and for selectively directing said beam ofelectromagnetic radiation following said third optical path into atleast tenth, eleventh and twelfth optical paths; each of said opticalpaths being of a different predetermined length; and means operablyassociated with said first and second means for controlling the pathtaken by said beam of electromagnetic radiation.
 28. An optical timeshifter and routing system as defined in claim 27 further comprising aplurality of additional beam receiving and directing means forpermitting said beam of electromagnetic radiation to follow apredetermined number of further different optical paths to anotherpredetermined location whereby, based on the combination of opticalpaths through which said beam of electromagnetic radiation passes, thetime said beam takes to reach said other predetermined location iscontrolled.
 29. An optical time shifter and routing system as defined inclaim 28 further comprising means interposed between a preselected pairof said receiving and directing means for suppressing unwanted signalsresulting from the passage of said beam of electromagnetic radiationthrough said predetermined combination of said optical paths.
 30. Anoptical time shifter and routing system as defined in claim 27 whereineach of said receiving and directing means comprises a diffractiveelement controlled by an electrical signal.
 31. An optical time shifterand routing system as defined in claim 30 wherein said diffractiveelement comprises a cascaded multiple grating in a single stage.
 32. Anoptical time shifter and routing system as defined in claim 29 whereinsaid signal suppressing means is in the form of a saturable absorberusing nonlinear absorption to heavily attenuate said unwanted signalswhile imposing negligible attenuation to the desired signal passingtherethrough.
 33. An optical time shifter and routing system as definedin claim 29 wherein said signal suppressing means is in the form of asaturable absorber using nonlinear absorption in quantum-well saturableabsorbers to heavily attenuate said unwanted signals while imposingnegligible attenuation to the desired signal passing therethrough. 34.An optical time shifter and routing system as defined in claim 31wherein each of said cascaded multiple grating comprises a pair ofgratings fabricated with independent diffraction angles.
 35. An opticaltime shifter and routing system as defined in claim 27 wherein saidoptical paths are created in free space devoid of external boundaries.